1. Field of the Invention
The present invention relates in general to a power supply, and more particularly to an asymmetrical duty cycle flyback converter for miniaturization and in power efficiency.
2. Description of the Prior Art
Generally, a power supply comprises a transformer connected between an input stage and an output stage, and a switch connected between the input stage and the transformer. The switch is on/off-controlled by a pulse signal supplied from a pulse width modulation integrated circuit (PWM IC). If the switch is turned on, current flows therethrough. When the switch is turned off, a voltage is applied across it while current flowing thereto is blocked. As a result, the current and voltage are overlapped, resulting in a loss in power.
A technique is known which designs the switch in such a manner that the current and voltage are not overlapped and controls the on/off operations of the switch, so as to improve a power efficiency. Such a technique is so-called "zero voltage switching (ZVS)".
FIG. 1 is a circuit diagram of a conventional half-bridge converter for the ZVS. As shown in this drawing, the half-bridge converter comprises a transformer T for transforming an input voltage Vi at a desired ratio, a switching circuit 10 for switching the input voltage Vi to the transformer T, and a rectifying/smoothing circuit 12 for rectifying and smoothing an output voltage from the transformer T. To this end, the rectifying/smoothing circuit 12 includes a first diode D1, second diode D2, coil L and a second capacitor Co.
The half-bridge converter further includes a controller 14 for controlling the switching operation of the switching circuit 10, and a driver 16 for driving the switching circuit 10 in response to an output signal from the controller 14.
The transformer T includes a first winding N1 at its primary side and second and third windings N2 and N3 at its secondary side, which are connected in series to each other.
The switching circuit 10 is provided with a first field effect transistor Q1 and a second field effect transistor Q2 connected in series between an input stage and a ground voltage source. The first winding N1 of the transformer T has its one side connected to a node between the first and second field effect transistors Q1 and Q2. A first capacitor Cs is connected in series to the first winding N1 of the transformer T.
The transformer T is provided with a half bridge at its secondary side, and, in the rectifying/smoothing circuit 12, the first diode D1 has its anode connected to the second winding N2 of the transformer T, the second diode D2 has its anode connected to the third winding N3 of the transformer T, the coil L has its one side connected to a node between the first and second diodes D1 and D2, and the second capacitor Co connected between the other side of the coil L and a node between the second and third windings N2 and N3 of the transformer T.
The controller 14 is preferably provided with a PWM IC. The driver 16 drives the first and second field effect transistors Q1 and Q2 in an asymmetrical and complementary manner in response to the output signal from the controller 14. To this end, the driver 16 is provided with an inverter 16a and a buffer 16b.
In the conventional half-bridge converter as shown in FIG. 1, the ZVS is realized by switching the first and second field effect transistors Q1 and Q2 in the asymmetrical and complementary manner. However, the above-mentioned conventional half-bridge converter has a disadvantage in that the transformer T is large in size because it is provided with the half bridge at its secondary side. Further, an output voltage from an auxiliary power supply (not shown), which supplies power to the controller 14 comprising the PWM IC, is varied with a variation in the input voltage Vi, resulting in heating of the controller 14 and the subsequent faulty operation thereof. The conventional half-bridge converter is further disadvantageous in that a switching noise occurs when the first and second diodes D1 and D2 are switched on/off.